Today, miniaturization has greatly increased the capabilities of many products, largely due to increased accuracy in the measurement techniques used during manufacturing. Similarly, reliability of many products has been increased, largely through increased repeatability of such accuracy in such measurements. Further, it has come to be generally accepted that even further increases in measurement accuracy and measurement repeatability are ways to even further increase the capacity and reliability of manufactured products.
One of the most accurate techniques used for measuring is interferometry. Interferometers operate by emitting radiation; splitting it into reference and measurement beams; permitting the reference beam to encounter a fixed set of conditions; permitting the measurement beam to encounter a variable condition; recombining the reference and measurement beams; and then studying the radiation wave interference effect that results to determine the extent to which the variable condition has actually varied.
In the basic interferometer three factors are variable: the wavelength of the radiation, the speed at which it travels, and the distance which it travels. To perform interferometric measurement two of these factors are held constant while the third is permitted to vary. Such variation produces corresponding change in the interference effect, which is measurable, thus making it possible to calculate the amount which the variable factor has actually varied. The accuracy and repeatability of measurements made in this manner are determined by the resolution of the instrumentation used, how well the constant factors are controlled and actually held constant, and how well any variation in the constant factors is later compensated for.
From the preceding it follows that three general types of interferometric measurement are possible: radiation wavelength measurement, radiation speed measurement (termed refractometry when light is used as the radiation), and distance measurement change (i.e., displacement). For displacement interferometry (e.g., for making physical measurements in manufacturing scenarios, which are the primary concern here) the wavelength and speed of the radiation are held constant, while the displacement of a target is permitted to vary and affect the measurable interference effect. To provide constant wavelength radiation today the overwhelming choice of radiation source is the laser. Lasers can provide light having suitable coherence (i.e., predominantly single wavelength, or at least quite discrete wavelengths), as well as light having known and repeatable wavelength. The speed of radiation is determined by the characteristics of the regions through which it travels. In a vacuum all electromagnetic radiation travels at the speed of light. But, through different materials radiation travels at slower speeds. For interferometers using laser radiation (i.e., light) such materials typically are glass optical elements and air or other gas mixtures. Finally, in the context of light radiation, the common terminology used to discuss the speed of light in such materials is "index of refraction," the ratio of the speed of light in a vacuum to its actual speed in the specific material. All further discussion of radiation speed here will use the term index of refraction.
Returning now to interference, it is the well understood combination effect when multiple waves intersect. Wave interference produces fringe patterns, and as characteristics of the waves change the fringe patterns produced also change. For purposes of displacement interferometry the intended variable wave characteristic is the change in the distance which the radiation waves travel (i.e., the displacement). The mathematical relationship of such change is that one full cycle of fringe change occurs for each path length change of one-half radiation wavelength.
Suitable sensors, such as high-speed photo diodes, can be used to detect the changing intensity of fringes falling upon them. In displacement interferometry this is exploited in two manners. For coarse measurements, the changing fringe cycles may be counted as they pass a sensor. For fine measurements, the phase of particular fringe cycles may be detected (with .lambda./64 resolution common in instrumentation used today). In theory, using 633 nano meter wavelength radiation from typical HeNe lasers, course measurement resolution of .+-.3.times.10.sup.-7 meter, and fine measurement resolution of .+-.1.times.10.sup.-8 meter are possible. Further, using other techniques, such as orthogonal polarization phase shift doubling (today, an entirely conventional practice, which is not particularly germane to the following discussion), even these resolutions can theoretically be bettered. Unfortunately, theory and practice often differ by orders of magnitude, and laser displacement interferometry today is very much an example of this.
As interferometry has matured the following terms have come into general use. Resolution is the minimum detectable displacement of a target. Dynamic range is the ratio of measurement range to resolution. And, precision is the resolution relative to the physical size of the instrument. Basic interferometry in manufacturing environments today is capable of dynamic ranges of 1.times.10.sup.-5 (e.g., resolution of 3.times.10.sup.-6 meters across the diameter of a 300 mm semiconductor wafer). With environment control, dynamic range can be extended to 1.times.10.sup.-7. And, with environmental compensation the inventors are able to obtain dynamic range approaching 1.times.10.sup.-8.
Two sources of error in laser displacement interferometry are common. First, the constant factors of wavelength and index of refraction are never absolutely constant. Second, the instrumentation which is used has resolution limits of its own. Today, light wavelength concerns are usually addressed by improvement in the frequency stability and the repeatability of the laser radiation sources used. And, while quite important in interferometry, this class of errors is not of concern here. Similarly, general instrumentation design is not of concern here, being merely an exercise in engineering. Neither of these error sources will be discussed further herein.
The present interest is errors caused by changes in the index of refraction of the materials through which laser interferometer radiation travels. For purposes of this discussion these refractive materials can be classified as the general optics of the interferometer, and the air-filled regions which the laser beams travel through on their way to and from retro reflective targets (plural, in some implementations, since the reference beam reflector is also really a target, and may have either an optical glass or an air path to it as well). The refractive indexes of the general optics can be made relatively stable, can be measured, and can be compensated for with relative ease. Unfortunately, both determining and controlling the index of refraction of air are not such easy tasks.
The refractive index of air is a function of pressure, temperature, humidity, and gas composition (dealt with by most writers as merely CO.sub.2 concentration). (See generally, Bobroff, Recent Advances in Displacement Measuring Interferometry, Meas. Sci. Technol. 4 at 907-26 (1993); and Estler, High-accuracy Displacement Interferometry in Air, Applied Optics Vol. 24, No. 6 (Mar. 15, 1985).) Most text book discussions deal with these variables as static influences on the index of refraction. Unfortunately, as reference to any weather report will illustrate, air is not necessarily static nor suitable for study as merely a combination of four static variables. Each of these factors, individually and in combination, may further vary considerably across distance and time (i.e., vary dynamically). And, experience is showing that even in very small manufacturing environments, such dynamic effects on the refractive index of air can appreciably affect interferometric measurement accuracy and repeatability. (Estler at 809-12 extensively discusses amounts of change in refractive index due to atmosphere dynamics, as well as the equations today felt to govern such change.)
As somewhat alluded to previously, two approaches can be taken, individually or in combination, to reduce errors caused by changes in the refractive index of air. First, the characteristics of the air may be controlled. Unfortunately, this can be difficult, e.g., maintaining constant pressure as a storm front moves through; or, even counter-productive, e.g., slowing production equipment to reduce turbulence as 300 mm semiconductor wafers are moved in a chip fabrication process. Further, since movement of tools and work pieces is inherent in most manufacturing process, environmental control often has manufacturing process imposed limits in addition to inevitable environmental engineering limits. However, aside from noting that the inventor's techniques are also quite suitable for application to study atmospheric conditions when refining control techniques, control techniques will not be discussed further here. The primary concern here is the second approach, performing correction of measurements for environmental changes, an approach which the profession has come to term "compensation."
Compensation, in the present context of manufacturing metrology, is a relatively new and evolving science. For example, while empirical equations for pressure and temperature related compensation have been derived and are today considered well correlated, gas composition is rarely considered (and then, as noted, usually only in relation to CO.sub.2 concentration). Of particular current interest is humidity, because until quite recently measurement accuracy for relative humidity in non-laboratory conditions was typically only 5-10% (however, 1% and better accuracy is now becoming obtainable in automated systems). Further, due to the empirical nature of the equations used for compensation, there has been considerable ongoing refinement of these equations, particularly regarding humidity. The net result of this has been that, outside of research and academic laboratories, humidity has until now been largely neglected as a measurable factor, and thus also as a controllable, and compensate able one. However, driven by the need for even higher precision in measurements, to accomplish even greater product capability and reliability, humidity is now drawing serious attention.
As noted previously, the study of air index of refraction has static and dynamic aspects. Compensation is proving to be an art which can be refined considerably by applying this fact. The characteristics of air that determine its refractive index are not necessarily static, across both distance and time these characteristics can be quite dynamic. For example, it has already been noted that pressure is time dynamic as storm fronts pass. And, similarly, temperature may be highly location dependant, surface infrared emission effects being one example, and air stratification effects another (see e.g., Estler at 810, discussing the use of multiple averaged temperature sensors for compensation of the later). Further, even human presence in the manufacturing environment can affect the refractive index of air. Bobroff, at 916, discusses human contributions to CO.sub.2 concentration. And, obviously, human breath has a considerable localized humidity affecting potential. In many manufacturing scenarios all of these above example influences can, to some extent, be addressed with either control, compensation, or both.
Unfortunately, some manufacturing scenarios are not easily subject to rigorous environmental control. For example, as also previously noted, due to wafer movement during semiconductor manufacturing the characteristics of the surrounding air can be dynamic. Further, the inventors have observed that merely changing the axis of measurement may appreciably change the refractive index encountered. Further, air dynamics may even be a desired part of the manufacturing process. For example, laminar air streams are deliberately created and used in some clean room environments to flush potential contaminants away from critical manufacturing process regions. Unfortunately, most compensation today utterly fails to take such air dynamics into account, and compensating systems, with limited exceptions, are little more than home weather units hung on a convenient wall and electronically patched and software kludged into the interferometer or process electronics.
Another area of needed compensation improvement can be termed usability improvement. For example, merely measuring humidity, pressure, and temperature with sensors alone does not accomplish compensation. Some form of calculation must be made based on the sensor measurements, and then used to correct the laser interferometer displacement measurements. This can be done either by a human or automatically, preferably at the measurement stage (i.e., in the interferometer system), but if necessary within the process controls. However, this later method forces multiple tasks upon the process controls, typically requiring the steps of acquiring sets of inputs from the sensors and the interferometer, interpreting them, combining to calculate a true displacement, and then passing on the result; with the displacement being the only value actually germane to the manufacturing process. Unfortunately, to date, automatic compensation techniques have required users to themselves combine laser interferometers, atmospheric sensor equipment, and their respective manufacturing process--at least three separate sub-systems. For users this has proven to be awkward and error prone. Thus, a key observation of the inventors is that users want a single integrated measurement solution, to add to their individualized manufacturing processes. Another key point is that users want a measurement solution that integrates well with their manufacturing processes. To date, typified by general origins as weather forecasting equipment, atmospheric sensors have been large awkward discrete sensors and systems. And it follows that they have been more suited to studying large volumes of air, rather than localized points in the manufacturing processes of small products.